Air conditioning is one of the coolest benefits of living in modern times. Yet the amount of energy it takes to cool a building is staggering — in the U.S., for example, 15 percent of the electricity used by buildings is dedicated to cooling them. A new, super-thin, mirror-like material developed by a Stanford University team may help rethink how we air-condition our spaces, reports The Economist.
Under Stanford University electrical engineering professor Shanhui Fan and research associate Aaswath Raman’s proposal, outlined in the publication Nature, buildings would release the heat generated into outer space, rather than use pumps and compressors to address the heat produced when cooling a space. Such a system would have a notable effect on the environment. As the researchers note, “A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption.”
Long overdue updates on air conditioning technology will save energy. From Wayne Wilkinson.
So, how does their solution work? Outer space, which is a frigid 3 degrees Celsius above absolute zero, provides the “perfect heat sink” — our planet radiates heat into it constantly, which is “compensated for by the heat the planet receives from the sun.” But how would we apply that same principle to a particular building? “To encourage one part of Earth’s surface (such as an individual building) to cool down, all you need to do in principle is reflect the sunlight which falls on it back into space, while also encouraging as much radiative cooling from it as possible.”
The researchers created a material that reflects 97 percent of sunlight while radiating at a wavelength between eight and 17 microns, “which is where the atmosphere is most transparent,” explains The Economist. The material is composed of four layers of silicon dioxide and three layers of hafnium dioxide and also includes silver, which functions as a mirror. The result is a material that “does to light what a semiconductor does to electricity.” In other words, it manipulates light’s energy levels.
The researchers positioned this material on a silicon wafer placed within a specially designed box for minimizing heat conduction, then placed the box outside in “wintry” day in California. The material “settled down to a temperature 4.9°C cooler than its surroundings.” If the box were connected to its warmer surroundings, “that temperature difference would disappear,” and the material would, in effect, cool its surroundings.
“This is very novel and an extraordinarily simple idea,” Eli Yablonovitch, professor of engineering at the University of California, Berkeley, told Stanford. “As a result of professor Fan’s work, we can now [use radiative cooling], not only at night but counter-intuitively in the daytime as well.”
Some of the hurdles to be addressed include replacing the existing model’s expensive materials with inexpensive alternatives, and developing it for a much greater scale. While it doesn’t present an end to traditional air conditioning — it would only be able to be implemented on roofs of buildings with an unhindered view of the sky above — it is a promising move to reduce energy impact.